This presentation reported on a study that evaluated the potential for the biodegradation and associated reduction in aquatic toxicity of oil sands process-affected waters (OSPWs) using flow-through laboratory wetland microcosms. Changes in the composition of naphthenic acids (NAs) over a period of 52 weeks were also identified. OSPWs from Syncrude and Suncor were used in the experiments. The 2 types of OSPWs were enriched with nitrogen and phosphorus and had short and long hydraulic retention times (HRTs) of 40 and 400 days. HPLC/QTOF analysis was used to track changes in NA mixture profiles in each treatment over time. The biodegradation of NAs in Suncor OSPW was considerably faster than that of Syncrude OSPW. The biodegradation of NAs in both sources of OSPW was enhanced under longer HRTs, but the influence of nutrient addition was minimal. NAs that had the lowest degrees of cyclization and lowest carbon number were found to degrade faster, which is consistent with previous trends observed for aerobic microbial degradation of NAs using laboratory incubations. The 3 most persistent fractions of NA homologues were also identified within the NA mixture fingerprint, which may explain the lack of correlation between the mostly unchanged toxicological response as measured by Microtox and the 78 per cent reduction in total NA concentration over the study period.